Step-counter devices are known, which, worn or carried in some other way by a user, enable measurement of the number of steps taken and calculation of the distance covered, in addition to supplying further information, such as for example average speed, or consumption of calories.
Step-counter devices are, for example, used in inertial navigation systems (the so-called “dead reckoning” systems) applied to human beings, which enable tracking of the movements of a user, by identifying and measuring his/her displacement starting from a known starting point, without resorting to the use of a GPS (Global Positioning System) system, or acting as an aid to a GPS system. In these systems, a compass supplies the information related to the direction of displacement, and the step-counter device supplies the information related to the amount of displacement.
Step-counter devices are moreover used in a wide range of applications in the clinical field (for example, in rehabilitation), and in general in the fitness field (as instruments for monitoring the physical activity performed by the user).
In particular, step-counter devices are known that use integrated accelerometers of a microelectromechanical (MEMS) type for step detection.
In particular, these devices are particularly compact, so that they can be conveniently worn by the user, or advantageously integrated within portable electronic apparatuses, such as mobile phones, smartphones, PDAs, tablets, audio digital players, photographic cameras, or video cameras.
The aforesaid step-counter devices implement a step detecting method based on the analysis of the plot of a vertical acceleration that is generated during the various phases of the gait on account of resting of the foot on the ground, and that is detected by the accelerometer fixed with respect to the user body. In particular, execution of a step is determined by identifying acceleration peaks that arise in the acceleration signal, and these peaks are detected by comparing the acceleration signal with a given reference threshold.
Notwithstanding the use of MEMS sensors, with low energy consumption, known step-counter devices require a supply source, typically a battery source, for their use and for the transmission of the information detected to external devices (for example, a portable electronic apparatus, such as a smartphone).
It is also known that systems for energy harvesting from environmental energy sources arouse today considerable interest in a wide range of technology fields, given the increasingly widespread requirement of miniaturization of electronic systems, in particular portable ones, and the corresponding need for supplying these electronic systems with batteries of small dimensions or, where possible, with their own supply sources (so-called “self-supply”).
Typically, energy-scavenging systems are designed to collect, store, and transfer energy generated by mechanical sources to a generic electrical load. The mechanical energy is converted by one or more transducers (for example, piezoelectric or electromagnetic transducers) into electrical energy, which can then be used, after appropriate conversion and processing, to supply the electrical load. In this way, the electrical load itself does not require batteries or other additional supply systems. For example, low-frequency vibrations, such as mechanical vibrations of disturbance in systems with moving parts, may constitute a valid energy source.
In particular, systems have been proposed for energy scavenging starting from human walking, by means of appropriate energy-scavenging systems housed inside, or coupled to, the shoes of a user.
In this regard, the following documents (incorporated herein by reference) may, for example, be cited:
“Shoe-mounted PVDF Piezoelectric Transducer for Energy Harvesting”, D. Fourie, MIT Undergraduate Research Journal 19, pp. 66-70, Spring 2010; or
“Energy Scavenging with Shoe-Mounted Piezoelectrics”, N. S. Shenck, J. A. Paradiso, Micro, IEEE, vol. 21, No. 3, pp. 30-42, May/June 2001.
These systems envisage, by means of piezoelectric transducers, recovery of part of the mechanical energy dissipated when the foot rests on the ground during walking. The energy recovered may for example be used for supplying portable electronic devices, or for activating wireless transmission of information.
As illustrated in general in FIG. 1, an energy-scavenging system of a known type, designated by 1, comprises: a transducer 2, for example of a piezoelectric type, subjected during use to vibrations or other environmental mechanical stresses and configured for converting mechanical energy into electrical energy, typically into AC signals; a scavenger interface 4, for example comprising a diode-bridge rectifier circuit, configured for receiving at its input the AC signals generated by the transducer 2 and supplying at its output a DC signal for charging a capacitor 5 connected to the output of the rectifier circuit; and a voltage converter or regulator 6, for example a DC/DC converter, connected to the capacitor 5 for receiving at its input the electrical energy stored by the same capacitor 5 and supplying it to an electrical load 8. The capacitor 5 has hence the functionality of energy-storage element, to store energy, which is made available, when required, to the electrical load 8 for its operation.
The energy efficiency of the energy-scavenging system 1 is evaluated as a function of the ratio between the power supplied at input by the transducer 2 (designated by PTRANSD), and the power supplied at output by the scavenger interface 4 (designated by PSCAV) or the power available on the load (designated by PLOAD).
In general, the need is certainly felt to further improve step-counter devices of a known type, in particular as regards the corresponding electrical supply and the corresponding energy efficiency.